1. Technical Field
The present invention relates to optical gain media and, more particularly, to solid state gain media such as an optical fiber amplifier or cascaded optical fiber amplifiers to provide pulse outputs of high power in the kW to MW range and high energy levels in the mJ range. As used herein, solid state gain medium is in reference to a optical fiber amplifier or laser.
2. Background Art
Optical fiber amplifiers receive coherent light of relatively low power from laser injection sources and amplify the light to higher power. Such amplifiers have been used in fiberoptic telecommunications and cable television systems to boost the power of a modulated optical signal being transmitted along a fiber transmission line. Erbium-doped fiber amplifiers (EDFAs) have been found to be especially convenient for fiber communication systems because their amplification wavelength (near 1.54 .mu.m) is conducive to low loss propagation of optical signals in glass transmission fibers. Various U.S. patents describe such systems, and the fiber amplifiers used by them, including but not limited to U.S. Pat. No. 5,185,826 (Delavaux); U.S. Pat. No. 5,218,608 (Aoki); U.S. Pat. No. 5,218,665 (Grasso et al.); U.S. Pat. No. 5,331,449 (Huber et al.); U.S. Pat. No. 5,337,175 (Ohnsorge et al.) and U.S. Pat. No. 5,339,183 (Suzuki). Various forms of signal modulation are used in these systems. Laser diode signal sources are capable of providing 10 mW to 100 mW, single mode, modulated light beams at high modulation rates, typically greater than 10 MHz, with low modulated drive currents. The modulated signal can then be amplified to higher powers, most typically up to about 100 mW, via the fiber amplifier.
In U.S. Pat. No. 5,335,236 to Toeppen discloses a fiber amplifier which is injected with a seed beam and with a pulsed pump beam. The injected seed beam is generally continuous, but can in be pulsed, provided its pulse length is greater than that of the desired output pulse. The amplifier provides a pulsed amplified output whose pulse length is determined by the pump pulse length.
A number of potential applications for fiber amplifiers, including LIDAR systems, nonlinear frequency conversion laser printing, pyrotechnic applications and material processing applications (such as material cutting or marking), require higher power levels than those normally used for fiberoptic communications. For such applications, amplified pulsed outputs with high peak powers of at least 10 W and up to 100 kW or more and with high pulse energies of at least 1 .mu.J and up to 10 mJ or more are required for the best and most efficient operation. For example, higher frequency conversion efficiencies can be achieved in nonlinear conversion crystal devices if light input into these devices is supplied at higher peak powers with higher applied energy levels. High pulse repetition rates and high average power (on the order of 1 W or higher) are also desired. Such output pulses are usually achieved by Q-switched solid-state lasers, such as disclosed in U.S. Pat. No. 5,303,314 to Duling, III et al. and U.S. Pat. No. 5,128,800 to Zirngibl, which disclose fiber lasers that use Q-switching or gain switching mechanisms triggered by modulated or pulsed input signals to provide pulsed outputs. These types of systems, however, are large and complex. With the advent of EDFAs, attention has been directed to achieving large power and energy pulse outputs from these less complex optical amplifiers providing high power levels with high energy levels in the mJ range, such as exemplified in the article of B. Desthieux et al., "111 kW (0.5 mJ) Pulse Amplification at 1.5 .mu.m Using a Gated Cascade of Three Erbium-Doped Fiber Amplifiers", Applied Physics Letters, Vol. 63(5), pp. 586-588, Aug. 2, 1993. At low input repetition rates, peak output powers of 111 kW and energies of 0.5 mJ at 1.5 .mu.m wavelength were achieved from a multimode fiber source. Thus, to achieve higher power levels and energies with less complex systems, multiple stage fiber amplifiers can be employed.
In an amplifying medium, such as a double clad fiber amplifier, a rare-earth doped double clad fiber having a doped core e.g., Nd.sup.3+ or Yb.sup.3+, is utilized. An injection source provides a signal for injection into the fiber core. Such a source is commonly a laser diode which is cost effective and most conveniently available. Typical power levels from an injection laser diode source may be in the tens of .mu.W to hundreds of mW. The fiber is pumped with a high power pumping source, such as an array of pump laser diodes, and its output is optically coupled into the inner cladding of the fiber. The double clad fiber amplifier output power levels can provide output power levels that reach into the 10's of kW or higher, but in order to do so, gains of 40 dB to 60 dB are required of the amplifier. However, the gain is usually limited to a range, such as between 30 dB and 40 dB, before the onset and resulting buildup of backward and forward amplified spontaneous emission (ASE), and backward Rayleigh scattering as well as other scattering noise developed in the gain medium that is scattered through out the gain medium and is propagating in the fiber core ultimately depleting the pump energy resulting in little or even no pumping power for the injection signal amplification. We collectively refer to all these types of noise hereinafter as "scattering noise". This limitation of gain, of course, limits the possibility of achieving higher peak power levels in pulsed operation of the fiber amplifier. Thus, the key to efficient amplification of the injection source signal to achieve a sufficient power and energy level is to saturate the fiber gain, minimizing the scattering noise. The problem to be solved is how this may be effectively achieved. In typical fiber amplifier configurations, the amplified backward scattering noise is the dominate loss mechanism for the pump source.
The problem of scattering noise can become more severe in cascaded, coupled fiber amplifier stages where backtraveling noise propagates from the previous stage so that it is necessary to provide a suppresser of some type between stages to reduce the amount of noise; otherwise, the achievable gain in a multistage amplifier will be limited that the amplification of the signal input will be so limited or partially nonexistent due to the amplified gain of the noise. Thus, as indicated in B. Desthieux et al., supra, in order to eliminate the noise gain, a saturable absorber is employed in the case of cw operated coupled amplifier stages. In the case of pulsed operated high power output coupled amplifier stages, a synchronously timed gate is employed between the amplifier stages to reduce the amount of possible backward and forward noise as a major contribution. The gate may take the form of an acousto-optic modulator or other modulator between the coupled amplifier stages to suppress this forward and backward traveling noise and provide more of the gain for amplification of the input signal via the initial stage amplifier. Such a gate adds significant cost to the optical amplifier system and leads to a less robust and bulkier architecture.
An object of the present invention is to provide an optical amplifier system capable of producing high peak power, high energy pulse outputs.
It is another object of this invention to provide an optical fiber amplifier system of one or more stages capable of achieving higher peak power, high energy pulse outputs by saturation of the fiber gain to minimize scattering noise.
It is a further object of this invention to provide an optical amplifier that more effectively provides for pulse outputs of higher peak power and energy without requiring any active synchronously timed gate between amplifier stages in order to reduce scattering noise, i.e., eliminates the requirement or consideration of any interstage gate or modulator.
Another object of the invention is to provide a method of operating an optical fiber amplifier in a pulsed mode that suppresses scattering noise to allow production of higher peak power, higher energy pulses.